The thermal spray coating of substrates with metals and metal alloys is a widely practiced method for protecting such substrates from corrosion or wear, or for introducing certain properties to the surface of such substrates. Thermal spray coating by the electric arc spray process is an efficient and economical method for applying protective layers to a wide variety of articles. Electric arc spray devices are well known in the art and a typical device is described in U.S. Pat. No. 3,546,415.
The application of corrosion resistant coatings of aluminum, zinc, aluminum-zinc alloys, and aluminum-magnesium alloys can be accomplished using the arc spray method. An article by H. D. Steffans entitled "Electrochemical Studies of Cathodic Protection Against Corrosion by Means of Sprayed Coatings" in Proceedings 7th International Metal Spraying Conference (1974) at p. 123 describes the arc spray application and corrosion testing of zinc, aluminum, and zinc-aluminum psuedo-alloy coatings. A review article entitled "Corrosion Prevention by Sprayed Metal Coatings" by S. Wiktorek and R. Ashbolt in Corrosion Australasia Vol 7, No.4 (1982) at p.9 summarizes the then state of the art in the field, and describes typical process conditions for electric arc spraying including the use of air as the usual atomizing and carrier gas in the arc spray gun. B. A. Shaw and P. J. Moran, in an article entitled "Characterization of the Corrosion Behavior of Zinc-Aluminum Thermal Spray Coatings" in Materials Performance November 1985 at p.22 discuss the arc spraying of aluminum and zinc wires of different diameters to form a pseudo-alloy coating of about 85 wt % zinc-15 wt % aluminum, and review the physical and corrosion properties of the coating. An article by P. O. Gartland entitled "Cathodic Protection of Aluminum-Coated Steel in Seawater" in Materials Performance June 1987 at p. 29 reviews the arc spray coating of steel with aluminum-5 wt % magnesium and the performance of the coating in seawater.
Detailed information on the equipment and operating conditions for arc spraying of aluminum, zinc, and aluminum-zinc alloys is given in a series of Technical Data bulletins published by TAFA Incorporated, a major supplier of thermal spray equipment and supplies. Technical Data bulletin 1.9.1.2-01T (1986) gives such information on arc spraying of aluminum wire. Technical Data bulletin 1.9.1.2-02Z (1986) gives such information on arc spraying of zinc wire, and recommends the use of air as the atomizing gas at pressures between 45 and 70 psig. Technical Data bulletin 1.9.1.2-02A (1986) gives detailed information on arc spraying of zinc-aluminum wire, and recommends the use of air as the atomizing gas at pressures between 45 and 70 psig.
While air has been the most widely used atomizing gas for electric arc spraying, the use of inert gas for this purpose has been described in several applications. An article by H. Kayser in Thin Solid Films Vol. 39 (1976) at p.243 describes an arc spraying facility including an enclosure 7.times.13.times.3 meters high purged with argon and an argon-driven arc spray gun used therein for the spraying of titanium, tantalum, niobium, tungsten, and molybdenum. U.S. Pat. No. 4,232,056 discloses the arc spray coating of titanium and stainless steel with aluminum to produce porous surfaces for use in boiling heat transfer. Nitrogen, argon, and mixtures thereof can be used as atomizing gas in the arc spray gun. The use of nitrogen atomizing gas and an arc gun current of 85 amperes are specifically disclosed for applying the first coat of a two-coat layer of aluminum on stainless steel. U.S. Pat. Nos. 4,526,839 and 4,596,189 describe the arc spray application of aluminum, zinc, tin, copper, nickel, or their alloys to various substrates to form lithographic printing plates. Air, nitrogen, and ammonia are described as possible atomizing gases to be used at pressures between 40 and 120 psig. The reference states that it is preferred to use air at 80 psig to drive the arc spray gun.
The arc spraying of reactive metals using air as the atomizing gas can be inefficient, uneconomical, or unacceptable when the reactive metals are oxidized or vaporized to any substantial extent during spraying and when the resulting deposited coating has altered chemical composition and properties. When inert gases are substituted for air for atomization in arc spray guns, air from the surrounding atmosphere can be entrained to contact the stream of sprayed metal and cause excessive metal oxidation if proper arc spray gun operating conditions are not used. Optimum operating conditions for the inert gas arc spraying of such reactive metals are not known in the art, but are disclosed in the invention described below.